As is known in the art, a Frequency Selective Limiter (FSL) is a nonlinear passive device that attenuates signals above a predetermined threshold power level while passing signals below the threshold power level. A key feature of the FSL is the frequency selective nature of the high-power limiting: low power signals close in frequency to the limited signals are unaffected. In this sense, the FSL acts as a high-Q (>1000 demonstrated) notch filter that automatically tunes to attenuate high power signals within a narrow frequency band as illustrated in FIGS. 1A, 1B and 1C which illustrate the frequency selectivity of a typical YIG FSL; the frequency response of: an input to the FSL being illustrate in FIG. 1A, the transmission loss through the FSL being illustrated in FIG. 1B, it being noted that there is significant attenuation to the frequency components in the input signals having power levels above the predetermined power threshold level, PTH (FIG. 1A) while the frequency components in the input signals having power levels below the predetermined power threshold level, PTH pass through the FSL unattenuated (except for by the small signal losses (resistive losses, impedance mismatch, etc.) and output power spectra being illustrated in FIG. 1C, for multiple weak and strong signals. With FSL, the power threshold level is set primarily by the structure of a ferrite material. For example, single-crystal YIG material is a ferrite material that provides a lower power threshold than polycrystalline YIG, which is then lower than hexaferrite materials. The difference in power threshold between these materials is on the order of 10-20 dB, with single-crystal YIG providing the lowest of around 0 to +10 dBm. As is also known in the art, ferrite FSLs rely on the non-linear response of a magnetized ferrite material. Above a critical RF magnetic field level the spin precession angle saturates in the ferrite and coupling to higher order spin-waves starts to occur. RF energy fed to the FSL is coupled efficiently to spin-waves at approximately one-half the signal frequency and then converted to heat.
The threshold power levels for the onset of limiting range from <−30 dBm for magnetostatic wave FSLs to >40 dBm for polycrystalline ferrite in subsidiary resonance FSLs. The critical RF magnetic field is directly proportional to the spin-wave linewidth of the ferrite material. Liquid Phase Epitaxy (LPE) Yttrium-Iron-Garnet (YIG) is typically used because it has the narrowest spin-wave linewidth of all measured materials, on the order of 0.2-0.5 Oersted (Oe). This single crystal YIG approach provides the lowest insertion loss for weak signals, the highest-Q filtering response, and provides a power threshold on the order of 0 dBm—collectively making the material the most attractive for a wide variety of applications. A typical implementation of an FSL includes a strip conductor disposed between a pair of ground plane conductors in a stripline microwave transmission structure using two YIG slabs or films for the dielectric, as shown in FIG. 2, to couple the magnetic energy of the interfering signal into the magnetic material. Permanent biasing magnets are mounted to the sides, as shown, or may be mounted to the top and bottom of the structure. The strength of the magnetic field within the structure establishes the operating bandwidth of the limiter. An electro-magnet may be used in which case a wire, not shown, is wrapped around the entire structure to provide windings in a direction perpendicular to the stripline. DC current flows through the windings to provide a bias magnetic field. The bias is selected to establish the operating bandwith of the limiter. The slab thickness is generally 100 um or less because of the difficulty in growing thick YIG films, requiring stripline widths on the order of 20 um to achieve an input impedance Z0 matched closely to 50 ohms. This approach is simple to fabricate and provides adequate magnetic fields to realize a critical power level of approximately 0 dBm when using single crystal YIG material. One method of reducing the power level threshold of the FSL is to use a lower input impedance stripline (i.e., less than 50 ohms); however, at the cost of degraded return loss. Thus, when using a lower input impedance structure, an impedance matching structure is sometimes used to improve the impedance match; however, this technique reduces the bandwidth and increases the insertion loss of the FSL; the approach reduces the resistive losses associated with the transmission structure for weak signals, and slightly increases the magnetic coupling of the signals with the ferrite material.